table inheritance?

[Riraito]

is there any way to get a table to inherit the value(s) of another table?
for example if I have:
a = {1,2,4}

is there any way to create a new table b, which inherits all the values of a, without having to do each value individually ( b = { a[1], a[2], a[3] } )

[Tekkub]

you can do a deep copy, or there's some metatabe magics you can use to make it look up a value in a if that value isn't stored in b... I think it would be...

setmetatable(b, {__index = a})

might have that wrong tho, metatables always mess me up.

[Riraito]

Originally Posted by Tekkub setmetatable(b, {__index = a})

that did it! thanks

[Riraito]

ok so I tried doing it manually and it seemed to work fine, but when I ran it in my code, it still acts like Im doing a simple a = b; statement, and when I change one table, it changes the other

[Shirik]

Originally Posted by Riraito ok so I tried doing it manually and it seemed to work fine, but when I ran it in my code, it still acts like Im doing a simple a = b; statement, and when I change one table, it changes the other

a = b doesn't make a copy, it makes a reference. Changing one will change the other.

Depending on what you want to do, tekkub's answer may be workable for you:

b = setmetatable({}, {__index = a})

Alternatively you can copy all of the keys in a for loop. If you just want to copy raw data, however, the quickest solution, for a simple table, will be to use a metatable to reference it until it's changed. For more complicated solutions (specifically handling nested tables), try this code I just made up now (very minimally tested!)....

-- Generates the metamethod to handle copying data
function _GenMetamethod(table)
   return function(t, k)
      if not rawget(t, k) then
         local v = table[k];
         if not v then return nil; end
         if type(v) == "table" then
            local newval = {};
            setmetatable(newval, {__index = _GenMetamethod(v)});
            return newval;
         else
            rawset(t, k, v);
            return v;
         end
      else
         return rawget(t, k);
      end
   end
end

-- Now we have a table
a = {1,2,3,4,5};
-- We can copy it to b like this
b = {};
setmetatable(b, {__index = _GenMetamethod(a)});
-- Now let's try this...
b[2] = 10;
assert(a[2] == 2 and b[2] == 10);
assert(a[3] == 3 and b[3] == 3);

If you don't understand how or why this works, I recommend checking out Programmign in Lua (PiL) regarding metatables